In a rotary offset printing machine, it is well known to use an array of ink-dosing elements or slides arranged across the width of the printing machine for zonally regulating the density of ink printed on the printed sheet. The density of ink is increased or decreased by displacement of the ink slides under the control of an automatic ink feed control system. At the present time all of the major printing machine manufacturers sell automatic control systems for this purpose. A light-emitting diode matrix or bar graph display has been used in some of these systems for indicating the ink slide displacements.
In a known control system of this kind, as described in West German patent publication No. 3,147,312, a light pen is provided for entry of set-point data and control commands, and has a light receiver cooperating with the light-emitting diodes. For inputing each individual displacement value for a respective ink slide, a printer puts the light pen on the light-emitting diode corresponding to the value in the light-emitting diode row for the respective ink slide, and receives a light signal emitted by the light-emitting diode as a result of a short duration current pulse, the light signal being detected by an electronic control unit connected to the light pen and being evaluated for the adjustment of the respective ink slide. The light-emitting diode in proximity with the light pen is recognized by the fact that the light-emitting diodes receive the current pulses in a predetermined sequence. To enable the light-emitting diodes to be used for display purposes at the same time, the current pulses are so short in duration that the light-emitting diodes appear to the human eye to be non-illuminated or only weakly illuminated. Those light-emitting diodes which provide a display, on the other hand, are additionally given a relatively long current pulse which makes them appear to the human eye as brightly illuminated. The light pen is connected to operate only during the times of the short current pulses and cannot therefore detect the display pulses. A disadvantage of this kind of control system is that a light pen with a light receiver is required for entering set-point data and commands, and the light pen is connected to the control unit via a cable. The light receiver may, for example, be disturbed by stray light or reflections, and the cable can readily be damaged.